Commonly the working voltage (also as rated voltage) of activated carbon-based supercapacitors such as electrical double layer capacitors (EDLC) reaches 2.5-2.7V in organic electrolytes (reference is made to the products of e.g. Maxwell, Nesscap, Asachi Glass, Panasonic, Ioxus). Organic electrolytes are most often solutions of tetraalkyl ammonia salts in acetonitrile or organic carbonates. When carbon nanotubes are used as active electrode material, the applicable maximum voltage of 3.5V has been reported by Kenji Hata, Hiroaki Hatori and Osamu Tanaike in [PCT/JP2006/316137].
Prior art [Kobayashi Kotaro et al. EP1860673A1] also teaches that KOH-activated graphitic carbon electrodes, which possess relatively low specific surface area, can be used at the working voltage of 3.3V.
Activated carbons commonly used in supercapacitors are characterized as microporous or micro/mesoporous carbons, which are made by pyrolytical carbonization and consequent chemical activation of different carbon-rich organic precursors. Most common precursors are coconut shells, resins and waxes, sugar, residues of a paper industry, etc.
Micro and mesoporous carbon materials also may be derived from metal carbides. Such carbons are categorized as mineral amorphous carbons and are often called as carbide-derived carbons (CDC). These carbons are made by chlorinating the metal carbide powder at temperatures between 200 to 1000° C. Due to crystalline highly ordered carbon precursor, the CDC materials possess high apparent density and uniform pore structure. A narrow, well controlled pore size distribution of CDC material has a key role in a superior electrical double layer capacitance of CDC-based electrodes that exceeding of 90 Farads per cm3 in organic electrolyte [Leis et al. Carbon 44 (2006) 2122-2129]. Despite of high capacitance, the working voltage Uw of CDC-based supercapacitors is never reported more than 3V, but usually 2.7-2.85V. The reasons of limited working voltage for the micro/mesoporous carbon based supercapacitors are decomposition of impurities in electrode material and absorption of electrolyte components in the carbon that promote an intensive heat and gas generation at voltages over 3V.